Apparatuses for collecting and removing solid particles from a flowing fluid are commonly known. They are used in sewage treatment plants in which domestic or industrial sewage is cleaned. They are also used to clean washing water or process water or to screen mud or the like.
An apparatus for collecting and removing solid particles from a flowing fluid is known from European Patent No. EP 0 221 991 B1. The apparatus includes a first set of stationary bars and a second set of movable bars. The stationary bars and the movable bars are alternately arranged, and they include steps at their side facing the flowing fluid. The steps include a substantially horizontal skeleton portion and a substantially vertical rising portion in a way that the steps form a rectangular stepped curve. The movable bars are interconnected to form a movable bar unit being driven along a closed circuit with respect to the stationary bars. The movable bar unit is driven by two eccentric discs in a forced manner. The upwardly directed component of movement of the circuit is slightly greater than the height of the steps of the stationary bars. The movable bars form a unit with side walls. The side walls also serve to transmit the circular movement to the movable bars. The bars including the rectangular steps are arranged such that they are declined. In other words, the apparatus is arranged in the fluid at a small inclination angle. This means that the apparatus has a comparatively great length in the flowing direction of the fluid. FIG. 6 of the European Patent No. EP 0 221 991 B1 illustrates a more steep arrangement of the bars. The bars include undercuts being arranged in the skeleton portion and in the rising portion. Thus, the steps include corners or tips protruding in a forward and in an upward direction. Again, the movable bar unit is guided along a circular path. The upwardly directed component of movement is a little bit greater than the height of the steps of the bars. The diameter of the circuit is substantially greater than the distance between two adjacent corners of the steps. The solid particles to be removed from the fluid tend to be squeezed, pushed or dislocated in an undesired way when they are transported. Not all solid particles are deposited on the next upper step as desired.
Another similar separating grid is known from the European Patent No. EP 0 682 551 B1. The separating grid includes a stationary bar unit and a movable bar unit. A driving mechanism is arranged above the flowing fluid to move the movable bar unit along a closed and substantially circular path of movement. The movement includes a vertical component of movement the value of which is greater than the height of the steps of the bars. The steps of the bars do not include any undercuts, but their surfaces form an angle being a little bit above 90 degrees. Some of the steps are arranged to be declined in a downward direction. The drive for the movable bar unit is designed and arranged such that the movable bar unit in its upper region is exactly guided on a circular path. The circular movement is provided by an eccentric device. The drive also includes a pulling element with a connecting mechanism providing a path of movement being similar to a circuit. In this way, the movable bar unit at each point is differently moved about the height of the bars. The path of movement is closed, and it is very similar to a circuit. The circuit includes a vertical component of movement being more than the height of the steps of the bars. With this arrangement, the solid particles are not transported in an upward direction in a way as desired. The upward movement of the particles is disturbed by pushing effects.
A step screen is known from German Patent Application No. DE 197 14 089 A1. The step screen includes a stationary first bar unit including stationary spaced apart bars and a movable second bar unit including movable bars. All bars include a majority of steps at their surface facing the flowing fluid. The steps include a horizontal skeleton portion and a vertical rising portion. The steps do not include any undercuts. The second bar unit is driven along a closed path of movement in the plane of main extension of the bars. The upwardly directed component of movement of the path of movement is slightly greater than the vertical height of the steps of the bars. The drive includes two separately controllable actuating drives driving the movable bar unit in two different actuating directions. In this way, the adjustment of the movable bar unit is more variable. The path of movement surrounds a surface, and it is divided into substantially linear sections of the movement, the sections preferably being vertical and linear. The actuating drives are arranged to be vertical and horizontal. Consequently, the circular movement known from the above described prior art is replaced by a rectangular movement. The separating grid has a relatively great length corresponding to the sum of the widths of the skeleton portions.
Such apparatuses known in the prior art require undercuts being provided at the bars to convey bigger solid particles, as for example cans. These undercuts being especially arranged in the rising portion of the steps result in an enlargement of the bars to attain the necessary stability of the bar unit. Additionally, undercuts require an enlargement of the radius of the circular path of movement along which the movable bar unit is guided. As a result, the solid particles to be collected and conveyed out of the flowing waste water are squeezed and crushed.